Respirators have long been used for purifying ambient air and for providing users with breathable air supplies in hazardous environments in which a user may encounter contaminated air. Such environments may present various types and concentrations of air contaminants. Different types of respirators have therefore been developed for providing different levels of protection.
Perhaps the most basic variety of respirator is an air purifying respirator (APR). The air pressure inside of a breathing mask that employs an APR is negative during inhalation with respect to the ambient pressure outside the mask. As a user inhales, air is drawn from the ambient atmosphere, through an air purifying filter, and into the mask. The user then exhales through an exhalation unit that typically includes a check valve that provides a relatively small exhalation resistance. The APR therefore resists the entry of unfiltered ambient air into the mask, but allows exhaled air to exit the mask with relatively little resistance. A problem commonly associated with APRs is that they can be susceptible to contamination if leaks develop in the respirator or between the mask and the wearer. APRs may therefore be sufficient for certain, low-contaminant environments, but are generally insufficient for environments the present relatively high levels of contamination.
A more protective respirator is a self-contained breathing apparatus (SCBA). SCBA units include an air tank containing compressed purified air. The tank provides positive pressure air to a respirator mask. Air enters the mask through a demand valve that opens when the user inhales. The cracking pressure of the exhalation unit check valve is greater than the cracking pressure of the demand valve to prevent continuous flow of air through the respirator. In this way, air flows into the respirator during inhalation but ceases to flow during exhalation. In addition to providing an independent source of pure air, SCBA respirators are advantageous in the continuous, positive air pressure inside the mask of a SCBA unit effectively prevents the ingress of ambient air. However, a problem commonly associated with SCBA respirators is that they can be relatively loud, and the source of air is limited to the volume of the associated bottle. Thus, the SCBA arrangement may not be optimal for all environments, especially those in which a user wishes to remain inconspicuous (e.g., for law enforcement and military use).
In view of the foregoing, it is apparent that if a user wishes to be prepared for different types of environments that may require different types of respirators, the user must carry and maintain multiple types of respirators. This can be very cumbersome and inconvenient. It would therefore be desirable to provide a respirator that can quickly and easily be converted for use in various operation modes, including APR and SCBA.